First-principles lattice dynamics calculations of the phase boundary between β-Si3N4 and γ-Si3N4 at elevated temperatures and pressures

✍ Scribed by Atsushi Togo; Peter Kroll

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 230 KB
• Volume: 29
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.21038

✦ Synopsis

Abstract

The phase boundary between β‐Si~3~N~4~ and γ‐Si~3~N~4~ is investigated at high pressure and high temperature using first‐principles lattice dynamics calculations within the quasi‐harmonic approximation. We find a positive slope of the phase boundary, hence, at higher temperatures it requires higher pressures to synthesize the high‐pressure polymorph of silicon nitride. It turns out that the thermal expansion of the spinel‐type γ‐phase is larger than that of the phenacite‐type β‐phase. On the other side, pressure affects more the volume of β‐Si~3~N~4~ than of γ‐Si~3~N~4~, reflected in the higher bulk modulus of γ‐Si~3~N~4~ up to about 40 GPa. The origin of the different temperature behavior of these phases, consequently, is rooted in a larger volume dependence of the zero point energy in γ‐Si~3~N~4~ in comparison to β‐Si~3~N~4~. © 2008 Wiley Periodicals, Inc. J Comput Chem 2008